Method of curving a flat molded plastic printing plate



Aug. 4, 1953 H. M. RICHARDSON ET AL METHOD FOR CURVING A FLAT MOLDED PLASTIC PRINTING PLATE Filed Feb. 25, 1949 Mw UM .@x W Q N Q AQ W1@ Y m .DOWN

Patented Aug. 4, 1953 UNITED sTATEs METHOD or cUnvING A FLAT MOLDED rLAsTIc PRINTING PLATE lHenry M. Richardson, William Baumrucke'r,

Springfield, Mass., and Jr., Stamford, Conn., as-

signorsto News Syndicate Co., Inc., New York,

N. Y., a. corporation of New York Application February, 1949, Serial No. 78,276 z claims.' (c1. 18-56) This invention relates to printing and aimS to provide an improved method of preparing 9e' molded plastic printing plate.

It is a particular object of our invention tofprovide a method of preparing plastic printing; 5 plates which can be practiced In the production: of newspapers. To satisfy the requirements or; newspaper production it is essential that the printing plates be produced at very high speed:

It is known that plastic printing plates are superior to the usual stereotype plates in they following respects: They have better wearing?- qualities andresistance to injury; they are lighten, in weight andyeasier to handle; they can be DIO- duced withvless; variations in thickness and sur-fV face grain; and they permit the reproduction otl engravings and type faces with a delity which,"` is not attainable with stereotype plates. Nevertheless, the uses to which molded plastic printing plates havebeen-put has been very limitedg Heretofore, it has not been possible to use theri successfully in the production of newspapers and commercial Vuse for other printing purposes has lbeen limited` by the slowness of production.

Plastic-printing plates have been made in the past by covering the face of a matrix with a powdered or vgranular thermoplastic material, The matrix used for this purpose is a hard rigidi,-r nat sheet whose face bears a reproduction in-jA intaglio of theqfaceI of a printing form. Th matrix bearing the `powdered thermoplastic ma: terial was plaed in a press Where it was sub jected to moderate pressure and heat vsufiicient 'f to melt the thermoplastic material so that it would now over the face of the matrix, pence-G5 trate the depressions in that matrix, and form l a sheet whose face bore an impression ofthe?` face of the matrix. The molded sheet was then cooled to produce a flat rigid printing plate This molding operation was too slow for gen eral use. A further limitation upon the use to which the flat vprinting plates produced by thisv method could be put resulted from the fact that r they could not be used easily on the cylinder of a rotary press.

`We have discovered that it is possible to produce a molded plastic printing plate at speedsg which fully meet the requirements of newspaper-. printingv and'other commercial uses, and that l sucha .printing platemay be curved to conform 50 to the curvature of the printing press cylinder for which it is designed.

A molded plastic printing plate is prepared by heating the face of a flat sheet of thermoplastic material to molding temperature. The heated face'of this sheet is placed against the face of a matrix and this assembly is enclosed in a sealed chamber between cooled platens of a molding press. The air is evacuated from that chamber and the sheet and matrix are then subjected to pressure between the platens of the molding press to mold the sheet into a printing plate. A flat plastic printing plate, preferably prepared as described above, is converted into a curved printing plate which will nt on the surface of a cylinder of a rotary printing press by heating the back of the plate until it is iiexible. The heated back of this plate is immediately pressed against the surface of a bending roll whose curvature is matched to that of the cylinderffor which the printing plate is designed. `While the plate is pressed against the bending roll it is cooled and hardened. The heated back of the plate is preferably forced against the surface of the bending roll by exerting a duid pressure on the face of the plate.

In order that the practice of our invention will be clear to those skilled in the art to which it pertains, we will describe `the specicmode of practicing it which is illustrated diagrammatically in the accompanying drawings in which:

Fig. 1 illustrates the steps which are performed in the moldingy operation; and

Fig. 2 illustrates the steps which are performed in the curving operation. v

In the method illustrated in the drawings, a printing plate is made from a sheet 5 of thermoplastic material about three-sixteenths of an inch thick. That sheet is composed-of a resinous Inaterial such as vinyl chloride copolymer resin which is hard and rigidpat ordinary room temperatures but which is suiliciently soft and plastic for molding at a temperature in the range of 250-3007 F.

The sheetA 5 is first conveyed through an oven'B where it is preheated to a temperature between 100 and 200 F. It `is then conveyed beneath a battery 1 of infra-red heating lamps vwhich are directed towardy the face of the sheet andA heat that face to molding temperature. After the face of the sheet 5 has been heated to molding temperature, a matrix B is placed face down on the heated face of sheet 5 and this assembly is immediately placed between the platens 9, I of a molding press, preferably of the quick-acting hydraulic type. A matrix which is particularly suitable is described in co-pending application, Serial No. 78,275, led by one of us on February 25, 1949. The press platens S, I0 are maintained at a temperature which is well below the molding temperature of Isheet 5,. Thisv is accomplished by supplying brine'or other coolingmedium to platen 9 through pipe II and by supplying a like cooling medium to platen I0 through pipe I2. .1. The press platen S is equipped with a compressible sealing gasket I4 and with. stops I5. 1 The 4 perature differential between the heated sheet 5 and the cooled platens 9. I0 which will permit the plastic material in that sheet to ow and be molded into a printing plate by such a pressure before it becomes hardened.

When the flat printing plate I8 is to be used on a rotary press it is curved after it leaves the hydraulic press to t the printing cylinder for which it is designed. To accomplish this, plate I8 is conveyed past a battery I9 of infra-red heatingglampswhich aredirected toward the back for the' plate.' IAS the piane- 'is passesme battery of linfra-red lamps I9, the face'of thev plate is chilled, as by jets of cool air supplied through press platen I0 is equipped with a-conduitA I6 which leads to a vacuum pump and receiver A(not shown). Immediately after thesheet 5 and fmatrix 8 are placed in the press, the platensl 8, Iillrzo The distance between the are forced together. closed platens, and the extent to which the matrix 8 is forced against sheet 5; is controiledb'y' the stops I5.

The compressible gasket round the sheet I5 and matrix 8 when they are in thepress to provide a sealed chamber which is formed as the platens are closed. 'As soon as this sealed chamber is formed, a valve (not shown) on pipe I6 is opened automatically and the sealed chamber is evacuated. This step prevents the entrapment of air between the faces of the sheet 5 and the matrix 3 and is of great importance in the practice of our method. We have found that the speed vat' which the heated plastic sheet 5 can be converted into'a printing plate I8 is a function of 'the temperature' differential between that heated sheet and th'e cooled platens 9, I0. `rI'hat temperature differential is governed in general by the temperature at which those platens are maintained, and its magnitude is limited by, and dependent upon, the molding pressure to which the sheet 5 and the matrix 8 are subjected between the platens 9;-I0. -When the temperature diiferential'between .the heated sheet 5 and the cooled platens 91-I 0- is increased by deceasing the temperature of the platens, the speed at which the sheet can be converted into a hard printing plate is also increased. The briefer the period Within which sheet `5 hardens in the press, the greater the molding pressure which must be applied to assure the necessary faithful impression of the faceof the matrix on the face of the sheet before the sheet becomes hardened. v

We prefer in molding sheet 5 into aprinting plate to maintain the temperature of the platens 9, I0 at about 50 F. The molding pressurefto which the sheet 5 and matrix il are then subjected is of the order of 3,000 pounds per square-inch. At such a pressure and temperature the heated sheet 5 can be quickly molded into a printing plate I8 while plastic and become hardened -before'it is removed from the molding press so that it retains the impression of the matrix face., This process can be completed in the extremely brief period of about ifteen seconds. l A 4 It is not essential that such an extremely high speed'be maintained to meet the requirements of every type of commercial printing. It will be satisfactory in certain types of .commercial printing if the printing plates are made 'at slower speeds such as can be attained by. subjecting sheet 5 and the matrix 8 to a lower molding pres.- sure '(of the order of 500 or more pounds per squareinch) and by maintaining the lowe'rter'n-` I4 is adapted:v to sur- D 4conduit 20. As a result, the back of the plate is raisedto-atemperature in the range of 160-250 F., Whilethe face of the plate is prevented from reaching a temperature at which it is soft enough fto i'ose theinipresfsi'on of the matrix face which has been moldedtherein. The plate I8 is thus renderedwsuigciently flexible to permit it to be wrapped around the surface of a bending roll 2l f 'vrhiclnrotates the direction indicated by the arrow..

'The bending roll 2| is of suitable curvature to match the printing press cylinder upon which the *printingplate 'I Sis designed to be used;l The e'xible plate 'I8 Ai's"c1"1vreyed'to 'that bending roll 2I and its leadingA edgefis' seized by the gripper 22. As the bending roll 2l revolves.' the back of the flexible plate i8"-is`"direc'ted against the surface of the bending roll by a roller 24. However, since'the molded face-ofthe printing plate is net of uniformheight such'a roller cannot exertthe uniform pressure which is necessary to cause all portiorisoflthe Aback of the plate to' iit snugly against the bendingroll.

Thefbackof the printing plate 8 is made'to 'co'ntormexactly with'the 'contour of the bending roll?` 2l' by exerting a -fiuid pressure againstthe .face 'off plate I8.- 'This is accomplished through a reciprocating box 25 which is recessed at 26. When the plate i8 is-'conveyed by the bending roll 2i to a poin't opposite thebox`25, the edges of the fbox are/forced 'against the blank margin providedon' plate I8 which then forms a cham- 4with recess 26. Compressed air is then suppl'iedto. lthat chamber through: pipe 21. The fluidpressure of this -compressed air against the face of the printingplate forces the back'of the plate uniformly against the surface of the bending-rollZI.'

The bending roll is maintained ata temperature of about F. througliatheating medium supplied to the interior of that roll through conduit 29; The temperature of the curved printinglp'late 23 is reduced' to' the'p'oint at which the material forming thel'plate 'is hard and rigid while'the printing plate 'hugs the-bending roll `2I and :the curvature imparted to the plate on'that bending roll isretained by the plate after itis removed fromuthe bendingroll.

The terms `which are used herein' are terms of description and not of limitation, and it is to be understood that modifications lmay be :made in the specific mode of practicing our invention which we have described without departing from the spirit ofour'invention'as it is denedin the appended claims.

what is claimed isi i l. The method of curving a flat molded plastic printing plateto t the cylinder of a rotary printing press which comprises heating the back ofthe molded sheetito render'- it 'flexible and simultaneously cooling-.the face of'said sheetfand 6 then pressing the back of said sheet against the References Cited in the le of this patent surface of a bending roll to curve and harden it. UNITED STATES PATENTS 2. The method of curving a. flat molded plasy HENRY M RICHARDSON. 2,442,333 Borkland June 1,

WILLIAM BAUMRUCKER, JR. 

